Since moisture-curable polyurethane hot-melt adhesives are free from solvents, various studies for use thereof as environmental-responsive adhesives have been conducted especially in the fields of fiber bonding and lamination of building materials. Such adhesives have been widely used also in industrial fields.
In recent years, in view of growth in demands for reducing the weight and thickness of optical components, using hot-melt adhesives instead of acrylic adhesives which have been mainly used has been studied in bonding of optical components.
An adhesive containing a hot-melt adhesive composition which is resistant to moisture and heat and composed of the following components, for example, has been disclosed as the above-mentioned adhesive (for instance, see Patent Literature 1): (a) 100 parts by weight of a polyurethane resin having a flow-beginning temperature ranging from 55° C. to 110° C., (b) 5 to 150 parts by weight of an unsaturated polyester resin having a Tg ranging from 0° C. to 110° C. and a molecular weight of 10000 to 25000, (c) 10 to 150 parts by weight of an epoxy resin having a softening point of 60° C. to 140° C. and a molecular weight of 700 to 3000, and (d) 10 to 200 parts by weight of an inorganic filler subjected to a surface treatment with a coupling agent, relative to the polyurethane resin (a).
Such an adhesive has a resistance to moisture and heat, and this resistance is at a practically acceptable level. In the case where an article in which the adhesive has been used for lamination is immersed into water, however, the water intrudes into the article in a relatively short time in some cases; unfortunately, the waterproof performance thereof was insufficient.
The hot-melt adhesive composition that is resistant to moisture and heat has an advantage of enabling adhesion even at low temperature; however, use of the epoxy resin causes problems such as insufficient flexibility of the adhesive itself and peeling of the adhesive layer from an article when the article is dropped.